1. Field of the Invention
The present invention relates to an apparatus and method for reproducing data from an optical disk.
2. Description of the Related Art
Related-art techniques are disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-114074, 2005-285293, 2006-120255, 2002-288839, and 2003-132533 and PCT Publication No. WO 2001/052249.
Recently, high-recording-density optical disks, such as “Blu-ray Disc (registered trademark)”, have been in practical use. As for non-cartridge type media, i.e., bare disks, recording surfaces thereof are often contaminated with fingerprints while being handled.
In the case where the recording surface of such an optical disk is contaminated with a fingerprint, reproducing light and recording light may scatter and attenuate in a contaminated area, so that reproduced signal quality and recording signal quality are remarkably degraded. At the worst, data may not be reproduced from the contaminated area.
When a portion contaminated with a fingerprint is irradiated with a laser beam in order to reproduce data, the detected amount of reflected light decreases, thus reducing the amplitude of a reproduced signal.
When data is recorded onto, for example, a phase-change optical disk or a dye-change optical disk whose recording surface is contaminated with a fingerprint, a laser beam for recording is disturbed by the fingerprint, resulting in power loss. Accidentally, the shape of a mark formed on the optical disk is disturbed. Disadvantageously, the degree of modulation is reduced during reproducing (playback), so that a reproduced signal in which the asymmetry is disturbed is obtained.
FIGS. 12A and 12B illustrate the waveforms of reproduced signals obtained as reflected light information during reproducing. FIG. 12A shows the waveform of a signal reproduced from an area contaminated with a fingerprint. FIG. 12B shows the waveform of a signal reproduced from an area that is not affected by a fingerprint.
As will be understood from FIGS. 12A and 12B, the amplitude of the reproduced signal waveform is significantly disturbed by the influence of the fingerprint. As the disturbance on the asymmetry, a center level of the amplitude of the reproduced signal also varies (an offset fluctuation in the center level).
To eliminate the influence of the fingerprint, signal processing shown in FIG. 11 have been performed. Specifically, a reproduced signal obtained as information contained in light reflected from an optical disk is supplied through an offset canceller 81 and an automatic gain control (AGC) circuit 82 to a decoder 83.
As the waveform of a reproduced signal obtained while an area contaminated with a fingerprint is scanned, a period during which the fingerprint affects the waveform occurs as schematically shown in FIG. 13A. This period will be referred to as “fingerprint period” hereinafter for the convenience of description. For the fingerprint period, the amplitude level decreases due to a reduction in the amount of reflected light. In addition, a center level of the amplitude varies. That is, an offset fluctuation occurs.
In this case, the offset canceller 81 including, for example, a high-pass filter removes a low frequency fluctuation to eliminate the offset of the center level as shown in FIG. 13B. Furthermore, the AGC circuit 82 performs amplitude adjustment (e.g., application of a gain according to an amplitude level) to obtain a reproduced signal waveform in which the influence of the fingerprint is eliminated as shown in FIG. 13C.
After the influence of the fingerprint is eliminated in the above-described manner, the decoder using, for example, PRML (Partial Response Maximum Likelihood) decodes the resultant signal to obtain a binary data sequence. After that, a decoding circuit (not shown) decodes the data sequence, encoded by run length limited encoding, and further performs error correction on the data sequence to obtain reproduced data.
A fluctuation of a signal, reproduced from a portion contaminated with a fingerprint, occurs in a higher frequency band than that in which a normal signal fluctuation is found within one rotation of an optical disk during reproducing (hereinafter, such a fluctuation will be referred to as “normal disk rotation fluctuation”). To improve resistance to a fingerprint in the circuitry shown in FIG. 11, therefore, the offset canceller 81 and the AGC circuit 82 are configured to function in a higher frequency band than that intended for a normal disk rotation fluctuation.